The problem is that my 1200 watt conventional heating element is creating a mechanical buzzing noise. Shown below is a schematic of my setup:
PD4 is the output from a 5v microcontroller and is triggered high based on an interrupt from a zero-cross over circuit.
AC hot out and AC neutral are connected to the conventional heating element. The triac is ultimately used to control the duty cycle of the heating element.
From what I've read, the mechanical buzzing is due to the high change in current vs change in time (di/dt) caused by varying the phase of the triac. The step in current can be as high as 15 amps.
Based on some research this problem sometimes surfaces as "light bulb ringing" in light dimming applications. Some solutions people have proposed is to use a choke inductor.
I setup a 39uH inductor in series with the hot load line thinking it would reduce the di/dt of the circuit however it made no change to the buzzing.
Best Answer
One method you might consider is to only trigger the triac at the beginning of half-cycles and then only trigger a percentage of half-cycles based on the desired heat output. If, for example, you wanted a 50% heat output, you would trigger the triac for 50 out of 100 half-cycles. For 25% output, you would trigger for 25 out of 100 half-cycles. You will need to take care not to make your trigger pulse too short or the triac may not turn on.
Similarly, you could change your opto-triac (MOC3023M) to a zero-crossing type (possibly MOC3043M) and then either use the above mentioned method, or simply time the on-time of the trigger for the desired percentage of on time, remembering that the coarseness of the control will be in increments of half-cycle times. By using a zero-crossing type opto-triac you no longer need your external zero-crossing detector.
For more on what I am describing (zero-crossing-mode) see: http://www.oztekcorp.com/blog/bid/45104/Controlling-Power-with-SCR-Phase-Angle-vs-Zero-Crossing-Mode
If you decide to use this method, you should probably get rid of the inductor as it is not needed and may cause problems with triac turn-off unless you add a snubber circuit. For more on this see: http://www.onsemi.com/pub_link/Collateral/HBD855-D.PDF
Refer to page 38 "WHY AND HOW TO SNUB THYRISTORS" and page 114 "TRIACs AND INDUCTIVE LOADS".